The present invention relates to a semiconductor laser device.
Conventionally, as semiconductor laser devices that continuously produce output of as high as 150 mW or more with emission wavelength of 780 nm, there have been those with the structure shown in FIG. 7. FIG. 7 is a view of an end face of a semiconductor laser device, showing layer structure of crystal of the semiconductor laser device, in which the layer structure extends in a direction vertical to a surface of the drawing sheet.
In FIG. 7, there are shown an n-type GaAs substrate 1, an n-type AlxGa1-xAs first cladding layer 2, an active layer 43, a p-type AlxGa1-xAs second cladding layer 4, a p-type GaAs etching stopper layer 5, a stripe-shaped p-type AlxGa1-xAs third cladding layer 6, an n-type GaAs current blocking layer 7 that forms a stripe-shaped notch portion, a p-type GaAs contact layer 8, an n-side electrode 11, and a p-side electrode 12.
The p-type GaAs contact layer 8 is formed so as to cover the p-type AlxGa1-xAs third cladding layer 6 and the n-type GaAs current blocking layer 7. The p-type AlxGa1-xAs third cladding layer 6 fills the stripe-shaped notch portion of the n-type GaAs current blocking layer 7. The current blocking layer 7 is composed of an n-type AlxGa1-xAs first buried layer 7a formed on the etching stopper layer 5, and an n-type GaAs second buried layer 7b formed on the first buried layer 7a. The term “stripe-shaped” refers to a narrow structure extending in a vertical direction to the surface of the drawing sheet. It is noted that the range of x is 0<x<1.
As the active layer 43, there is adopted a layer with so-called multiple quantum well structure. FIG. 8 shows details of the multiple quantum well structure. In FIG. 8, the vertical axis represents Eg (bandgap energy) of each layer that changes in response to a composition ratio x of Al, whereas the horizontal axis represents a distance from the substrate 1 to the layers. The active layer 43 has a well layer 43a called a quantum well. For the well layer 43a, a crystal is used which has an Al ratio smaller than those of barrier layers 43b and guide layers 43c, 43d on both sides. This makes Eg of the well layer 43a smaller than Eg of the barrier layers 43b and the guide layers 43c, 43d, which makes Eg of the well layer 43a look like a well. Also, a thickness of the well layer 43a is around 200 Å or less, that is equal to or sufficiently smaller than the de Broglie wavelength of electrons, so that the well layer 43a is called a quantum well. Also, the term “multiple” indicates that a plurality of well layers 43a separated by the barrier layers 43b are used.
The guide layers 43c, 43d are layers for adjusting the level of confinement of laser light in the direction of the horizontal axis of FIG. 8. For simplifying the manufacturing process, guide layers having a composition identical to that of the barrier layers 43b are often used.
According to the above-structured semiconductor laser device, carriers (electrons and holes) injected from the electrodes 11, 12 are confined in the well layers 43a, which are small in Eg, so as to be efficiently recombined. Also in the well layers 43a, high luminous efficiency is obtained by quantum effect. This provides advantages such as reduction of oscillation threshold current Ith, and increase of external quantum efficiency η. Here, the emission wavelength λ has relationship with Eg of the well layer 43a that λ is approximately equal to 1.4/Eg. It is noted that a unit of the emission wavelength λ is μm and a unit of Eg is eV.
If the oscillation threshold current Ith is low and the external quantum efficiency η is high, optical output P0 is equal to (Id−Ith)×η where Id is a driving current, so that large optical output P0 can be provided without considerable increase of the driving current Id. Therefore, larger optical output P0 can be achieved with the same driving current.
However, there has been a problem that a luminous layer of the conventional semiconductor laser device is made of GaAlAs, so that if oxygen and/or moisture exists around the semiconductor laser device, constituent Al atoms of the well layer 43a are photochemically reacted with the oxygen and/or moisture by optical energy of laser and oxidized, and thereby destroy crystal structure of the well layer 43a, which tends to deteriorate characteristics of a laser device.
One solution to this problem may be to use a crystal containing no Al in the well layer 43a. In the case of using the semiconductor laser device as a light source for optical disc systems such as CD-R (compact disc recordable) and CD-RW (compact disc rewritable), a laser emission wavelength of the semiconductor laser device should be approx. 780 nm.
FIG. 3 is a view showing composition of III-V-group quaternary mixed crystal In1-vGavAs1-wPw (0<v<1, 0<w<1) that is assumed as a prospective material of the above-described semiconductor laser device.
In FIG. 3, the vertical axis denotes a proportion of P in V-group elements in the mixed crystal, whereas the horizontal axis denotes a proportion of Ga in III-group elements in the mixed crystal. Four corners of the shown quadrangle indicate, clockwise from the upper right corner, GaP, GaAs, InAs, and InP, each of which is a binary mixed crystal. Also, four sides of the quadrangle indicate, clockwise from the right-hand side, GaAswP1-w, In1-vGavAs, InAs1-wPw, and In1-vGavP, each of which is a ternary mixed crystal. The inside of the quadrangle indicates In1-vGavAs1-wPw, a quaternary mixed crystal.
Solid line A in FIG. 3 is a line connecting compositions having the same lattice constant, a, of the crystal, i.e., the solid line A is a lattice constant line. Since the solid line A passes a lower right GaAs point, it represents a group of compositions that are lattice-matched with the GaAs substrate. Broken lines B1 and B2 in FIG. 3 are lines connecting compositions having the same Eg of the crystal, i.e., the broken lines B1 and B2 are Eg lines. The lines indicate that if, for example, a crystal whose composition is on the line B1 is used as a well layer, an emission wavelength of the semiconductor laser is approx. 780 nm.
In FIG. 3, the lattice constant a of the crystal becomes smaller as going toward upper right side in approximate vertical direction from the solid line A, and becomes larger toward the lower left side of line A. The Eg of the crystal becomes larger toward upper right side in vertical direction from the broken line B1 or B2, and becomes smaller toward the lower left side. Therefore, the emission wavelength becomes shorter toward the upper right side and becomes longer toward the lower left side.
As for the active layer of the semiconductor laser device, a layer in a portion high in laser light intensity preferably contains no Al. More specifically, not only well layers but also barrier layers preferably contain no Al. Therefore, for manufacturing a semiconductor laser with an emission wavelength of 780 nm, there may be used as a well layer an In1-v1Gav1As1-w1Pw1 (0<v1<1, 0<w1<1) crystal whose composition is on the broken line B1 of FIG. 3, and as a barrier layer there may be used an In1-v2Gav2As1-w2Pw2 (0<v2<1, 0<w2<1) crystal whose composition is on the broken line B2 of FIG. 3 and whose bandgap is larger than the well layer.
However, since there has been no clear guiding principle of combination of v1, v2, w1, and w2, such semiconductor laser has not yet been put into practice to date.